Integrated circuits and other semiconductor devices, such as, for example, a radio frequency (RF) power amplifier, a low-noise amplifier, or other like circuitry, may be created on a semiconductor substrate. An integrated circuit created on a semiconductor substrate is commonly referred to as an integrated circuit die. A semiconductor substrate that contains integrated circuit die is commonly referred to as a semiconductor wafer, or simply a wafer. Integrated circuit die is removed from a wafer by, for example, dicing the wafer in order to separate the integrated circuit die into individual pieces.
Integrated circuit die may be packaged in, for example, a plastic semiconductor package. The packaging process includes attaching the integrated circuit die to a die attach pad on a laminate substrate with, for example, an epoxy material. The laminate substrate is then overmolded with a plastic mold compound material, which forms the plastic semiconductor package. Typically, the die attach, molding, and curing of the plastic semiconductor package is performed under a high temperature environment. The differences in the coefficients of thermal expansion of the integrated circuit die, die attach epoxy, plastic mold compound material, and the laminate substrate result in thermal stress created during the heating and cooling phases of the packaging process.
Once the packaging process is complete, the plastic semiconductor package is shipped and delivered for final assembly, such as, for example, mounting the plastic semiconductor package on a circuit board of a wireless device. However, during the shipping and delivery of the plastic semiconductor package, and prior to final assembly, the plastic semiconductor package is subjected to various environmental conditions that expose the plastic semiconductor package to moisture. This exposure to moisture adds moisture content to the plastic semiconductor package. This in turn adds additional thermal stress to the plastic semiconductor package, over and above that caused by thermal-expansion coefficient differences, during high temperature processes. For example, any moisture content that is present in the plastic semiconductor package causes a greater thermal expansion at the die attach epoxy interface between the laminate substrate and the integrated circuit die.
This thermal expansion at the die attach epoxy interface produces an additional stress at the die attach epoxy interface. Since the laminate substrate is overmolded with a plastic mold compound material, this additional stress causes the die attach epoxy interface to crack or delaminate, during, for example, high temperature processes. This cracking or delamination of the die attach epoxy interface is undesirable.
Conventional laminate substrates typically utilize a gold plating process to form a finish on the outer layer of the die attach pad. However, while the gold finish provides for good electrical conductivity, it does not provide for good adhesion of the die attach epoxy interface. Consequently, the die attach epoxy interface is susceptible to cracking or delamination for this reason as well.
In an effort to overcome cracking or delamination at the die attach epoxy interface, prior art laminate substrates have tried to utilize better adhesion processes on the die attach pads, such as, for example, solder mask. However, the use of solder mask on the die attach pad is disadvantageous since, for example, solder mask is an extremely poor electrical and thermal conductor. The interface between the integrated circuit die and the die attach pad ideally should not adversely affect adhesion, electrical conductivity, or thermal performance. In addition, solder mask on top of the die attach pad increases the height of the integrated circuit die attached to the die attach pad and increases the electrical and thermal resistance of the die attach epoxy interface. This increases the probability of cracking and delaminating of the die attach epoxy interface and decreases the overall reliability of the plastic semiconductor package.